2.2 Testing system and procedure
All of the triaxial cyclic loading-unloading compression tests were performed in a RTX-4000 high-temperature and high-pressure rock triaxial testing system at the state key laboratory for geomechanics and deep underground engineering. The maximum axial loading capacity is 4000 kN, and the maximum confining pressure is 140 MPa. The triaxial tiered cyclic experiments were carried out under different confining pressure (σ 3) of 0, 10, 20, 30 and 40 MPa, and consisted of the following three steps. First, the confining pressure was applied at a rate of 0.4 MPa/s. Second, the specimens were loaded to the predetermined axial strain value at a controlled axial displacement rate of 0.04 mm/min, and then unloaded the deviatoric stress to zero at a rate of 40 MPa/min through controlling axial stress. In each subsequent cycle, the second deviatoric strain level was increased, and then unloaded it to zero again. Finally, the stress cyclic was continued in that way until the specimens enter the residual strength phase.